A media stream that is sent from one party to another party, may be sent directly between the respective parties' media termination point or via an intermediate media handling device. Herein forwarding a media stream is understood to encompass forwarding a media stream towards a party's user equipment—e.g. by an intermediate handling device—as well as to a user interface of the party's user equipment itself, e.g. by a communications application of the user equipment.
Applications are known per se in which is switched from forwarding a first media stream to forwarding a second media stream to a recipient. Such applications include personal greeting services and mid-call announcements. The switching from the first media stream to the second media stream may be of the head-to-tail type, wherein the second media stream is being forwarded upon termination of the first media stream. The switching from the first media stream to the second media stream may also include a period of overlap in which the two media streams are mixed in a mixer device (e.g. fade in/fade out) and the mixed media stream is forwarded to the recipient.
An example is a personal greeting service with greeting fade out. When a call is answered, the personal greeting (first stream) is mixed with the media of the destination party (second stream) and is then faded out over a short period of time, e.g. 3 s.
Another example is an in-call announcement with fade in and fade out. The announcement is faded into a media speech channel towards the served subscriber (for whom the announcement is destined). When the announcement is complete, it is faded out of the media speech channel towards the served subscriber. For the playing of a mid-call announcement towards one party receiving a first media stream in a Session Initiation Protocol (SIP) session, the Session Description Protocol (SDP) for that party is updated. The updated SDP has the effect that this party in the SIP session will expect a second media stream from another remote party, e.g. an announcement device.
The known methods present several drawbacks. The known switching from the first media stream to the second media stream has the inherent effect that there may be disturbance of media streaming towards the served subscriber. For instance, the arrival of media in accordance with the updated SDP will, in practice, not be synchronized with the moment that the respective UA starts listening to the new media stream.
Also the known use of a mixer device when switching from the first media stream to the second media stream has several drawbacks. Linking in a media mixer device into a speech path leads to a sudden increase in latency. While latency as such is directly resulting from the use of a mixer, the linking in of the media mixer device leads to a brief period of silence, since the media mixer device has to fill up synchronization buffers associated with the respective input media streams. For instance, if the synchronization buffer has a length of 50 ms, then the effect is a period of silence of 50 ms, before speech streaming continues. Further, when removing a media mixer device from the speech path, the media stream has to ‘catch up’. If the synchronization buffer has a length of 50 ms and this synchronization buffer is removed from the media stream path, then 50 ms of speech would be ‘dropped’ and the media stream is abruptly advanced by 50 ms.
These effects will lead to audible disturbance when switching from forwarding the first media stream to forwarding the second media stream.